Electrical equipment



y E. HOUGHTO/V A TTORNEY direct current.

`Patented Feb. 18, 1947 UNITED STATES PATENT "()Fl'lClr i v 2,415,323A 'l l i ELECTRICAL .EQUPMENT Edward W. Houghton, Chatham, N. J., assignmto Bell Telephone. Laboratories, Incorporated,- New York, N. Y., a corporation of New York n. Application August 22, 1944, Serial No. 550,666`

v1 Claim. (C1. 1v1-95) This invention relates to a Wheatstone bridge 'Y 'l embodying a thermistor arm for comparing al- 'ternating current power with direct current directly indicated values. f

Heretofore, a-Wheatstone bridgeembodying in one arm a thermistor material which is a resistance material varying'its effective `resistance greatly with temperaturajhas been utilized for comparing alternating 'current with Such bridge vis v normally balanced with direct current before the alternating current is applied to the`jhermistorarm.

After the alternating current hasube'cnapplied t0r such arm, the direct current is varied until bridge balance is restored.. This enables the de-v termination of the amount of yalternating current or voltage applied to the therrnistor arm.

rObviously the use of such bridge for comparing alternating current powerwith direct current power would involve computations to determine the amount of direct current power eiective at each bridge balance.

ing the computations wtha high degree of accuracy. This would tend to occasion errors Such bridge,A 'if contemplated for use in the factory or eld, would require trained. personnel'for n perform- The present invention is concerned with indicating directly the amount of direct current power used for comparison with alternating cur- 4 rent power in such manner as to obviateexten-Y )sive computations, except a mental 4simple arithmetical subtraction.

The object of the invention is to indicatey directly a known amount 'of direct current power with kwhich an unknown amount of alternating current power is to be compared.'

yoperation of In accordance with a specic embodiment asused with a Wheatstone bridge having avrnicroammeter connected across one'diagonal and embodyinga thermistor arm to which both alter-v nating and direct current power can vbe effectively applied, the present invention,comprises a resistance network calibrated in predetermined Steps of effective resistance for varying the direct current power' into the thermistor arm in directly indicated steps of, say, lmilliwatt over a. preselected range. In the absence of ambient heating effects,y the effective resistance kof the mlcroammeter indicates bridge unbalance at a,

secondk value of direct current power which is indicated in milliwatts on the calibrated network. The diierence between the two readings of the calibrated network in milliwatts represents di-r rectly the amount of .direct current power, say 1 milliwatt, causing the bridge imbalance at the microampere reading. This 100-minorili!d pere reading` alsozcalibrates the microammeter and can be varied from o to 10o to provide a direct freading of 1 milliwatt, or a fraction of 1 milli-I watt', on the microammeter.

`Next-the calibrated networkis adjusted to that position which causes the irst value of direct current power to be applied'to thethermistor arm and thereby thezero reading to be produced on the microarnmeter.` Then the alternating current power is supplied to the thermistor arm 'and adjusted until'the nrii'croammeter reading is made exactly 100 microamperes. As 1 milliwatt of direct current POWr was previously. y required to produce a similar bridgeV unbalance),v v

Y. and the consequent reading ofr100 microam peres on the microammeter, an equivalent."` amount of alternating current power or 1 Inilliiy watt, is being applied to the thermistor arm. l A- reading on the microammeter lying between 0 andv 100 indicates directly thata fraction of 1 milliwatt of'alterna'ting .power'isbeing applied tothe thermistor arm; andl a.; reading lyinbe`v tween 10o and 20o nucroamperes'indicates direcuy that 1 mi-uiwatt plus a fraction of 1 muliy watt of-alternating current y power is being ap plied to the thermistor arm.

Another method of comparing thedirect and.

alternating current powers concerns adjusting" the calibrated network to balance the bridge tori.` provide the zero readingon themicroammete'r in the manner above explained, and thereafter` adjusting the calibrated 'network to remove one or more milliwatts of direct current power irom the thermistor arm. This causes an oir-scale reading, below zero,.on the microammeter'. Then the alternating current power isapplied to the thermistor arm and adjusted `until the zero ofthe power sup-l Y thermistor arm.

reading is restored. The difference between the two readings in milliwatts on the calibrated network represents thel amount of alternating current power being applied to the thermistor arm.

A further method of comparing the direct and alternating current powers involves (l) Vadjusting the calibrated network vwhich also includes a calibrated potentiometer to balance the bridge and thereby establish the Zero reading on the microammeter when only the direct current' power is applied to the :thermistor arm, and (2) applying the alternating current power to the ther-mister armvand then adjusting both the calibrated network and potentiometer to balance the bridge and thereby'restore the zero reading to microammeter. TheV difference between the'- com- Vwith reference to Figs. 2 and 3 which illustrate certain fundamental considerations. Referring to Fig. 2, a Wheatstone bridge 5l! in the balanced condition is assumed to have an eiective resist# ance Rc which is equivalent to the resistance Ro' so that the power from source P is equally divided in a value Pn therebetween. Also the resistance bined readings of the calibrated network and po- Y tentiometer at each bridge balance represents the amount of direct current power applied to the The invention will be readily understood fromV the' following description taken together with the accompanying-drawing in which:

Fig. l is a schematicV circuit of a Wheatstone bridge measuring circuit adapted with a speciiic'y embodiment of the present inventionj and Figs. 2 and 3 are schematic circuit illustrations in simplifiedform of fundamental'principles em bodied in Fig. 1;

'V Referring to Fig. 1, comprisesresistance'arms I I, I2 and I3 of a xed resistancecharacteristic, land a resistance arm I4 embodying a 'thermistor provided with a temperature coeii'icient of resistance.

plug I1 is a source I8 of alternating current having an unknown value of power to be measured in accordance' with the present invention which will be hereinafter explained. A device I8a varies the power of the alternating current for afpurpose that will later appear. A double pole, 'double throw, switch 9 in 'its'upper position connects microammeter I9 in series with aY manually vari-V able resistor ISaVacross the horizontal diagonalv of the bridge I0 and in its down position con' nects both of them across the points `34 and'35.'

A source 240 of direct current energizes the thermister arm I4 under controlof a manually variable resistor 2 I.

In accordance jwith a specific embodiment the present invention comprises an attenuator V25 consisting of branches 26, 21,28 and 29, each including a plurality of xed contacts connecting in series a plurality of individual resistors of pre. lselectedvalues of resistance with which branches are associated movable arms 3U, 3| 32 and 33 respectively, mechanically connected together and manually operated as a unit. A junction point 34 ofthe arms 30 and 3| `is connected to one Y a Wheatstone Vbridge I0 Applied to the bridge arm I4 througha connectible jack I6 and f end of the variable resistor 2l which has its op-v posite endv connected to the positive terminal of`V the direct current source 20. The negative terminal of this source is connected to a junction point 35 of adjacent ends of leads 36 and 31 of which lead 36 has its oppositeend connected to the lower terminal ofthe vertical .bridge diagl onal, and' lead 31 embodying resistor 22 has its i opposite end applied to the lowermost contact of i V`resistance branch 28. Lead 23 'embodyinga resis'tor 24 is' connected between the lowermost'contY of resistance branch 26 and lead 31. The

blea'rins,` 32'l and 33 are joined through af which has a movable contact 38 Y Vu permost `cont ctofklthe re-Y values 'of the individual bridge arms is preselected such that the power Pn into the bridge 50 is further divided sothat one-'half of such power, or

a value is supplied to the load arm. Upon this basis the number of fixed steps of the total power P into the bridge 50 may be assigned as follows:

where P1 is the highest power stepand PN is the lowest power step. The difference in the values ,of the power between these steps is assumed to be ing the entire resistance of potentiometer RAy from the circuit, in so far as the resistor Rc is concerned.V When the movable arm of potentiometer RA is in its maximum position, the power from the source Pinto the resistor Rc is Plv-PA, This hasthe effect Yof inserting the entire resistancefvalue of potentiometer RA in the circuit, in

so farfas the resistor Rc is concerned. Thus the resistance Vvalue of potentiometer RA' is so chosen that as its movable arm is actuated from the minimum to the maximum position at each` step, Equation 1, the power into the resistor Rc is var- /ied linearly from the value PN to the value PN-PA. Thus the potentiometer RA serves to Vary linearly the power from the value PA' to Zero.

At the lowest power step PN the minimum resistance value of resistor Rian is zero. The resistance valueV of Ren should be so` chosen for each power step, Equation l,l that a movement ofthe adjustable arm of potentiometer RA from the minimum to the maximum position varies the power into the resistor Ro from the value PA to zero. The maximum resistance value of resistor Ren occurs at thehighest power step Pi. The resistor Ran has resistance Values equal to those of resistor Ran as above indicated.

With the movable arm of potentiometer RA in the minimum position the maximum resistance value of resistorRns occurs at the lowest power step PN; and minimum resistance value of re sistor Run occurs-at the highest power step Pi.

The resistance values of resistor Ron should be so chosen for the individual power steps, Equation l',` in the manner for selecting the resistance values of resistor Ren for the individual power steps as previously explained.

The resistance value of resistor Ren should be so chosen for the individual power steps, Equation 1, that the current drain on the source P is conchosen in the manner for selecting the values of.

Power steps as above resistor Rian for individual mentioned.

Thus in Fig. 3 the resistors Ran, Ron, Rian' and Rss constitute a manually variable attenuator provided with such steps of eilective resistance that the direct current power into the bridge 50, Fig. 2, or resistor Rc, Fig. 3, can be varied in a plurality of steps, Equation 1, with a constant difference between successive steps, Equation 2, and that at individual steps a movement of the adjustable arm of potentiometer RA from the minimum to the maximum position varies the power into the resistor Rc linearly from the value Ri to zero.

The circuit of Fig. 1 embodies the above-mentioned fundamental considerations of Figs. 2 and 3 such that the following elements are equivalent in the respective circuits and perform similar functions; Wheatstone bridges I0 and 50 `and resistor Rc; resistor 22 and resistor Rc; directy current sources P and 20; bridge arm load in Fig. 2 and thermistor arm I4 in Fig. 1; potentiometers RA and 31; and direct current attenuator comprising resistors Ren, Ron, Rian' and Ren and direct current attenuator 25 comprising resistor branches 26 (including resistor 24) 21, 28 and 29 and associated movable arms 30, 3|, 32 and 33 respectively. Thus are resistors Ran, Run, Ren' and Rian in Fig. 3 are individually equivalent to the resistor branch 26 (including resistor 24) and arm 30, resistor branch 21 and arm 3|', 'i resistor branch 28 and arm 32, and resistor branch 29 and arm 33 respectively.

In the operation of Fig. l for measuring the thereby in series with the source 20 and variable resistor 2|. 'Ihe resistor 2| is then manually adjusted until a preselected indication of.- sair 100 microamperes is produced on the microammeter I9. Thus a predetermined magnitude or standard amount of direct voltage is applied to the input of attenuator 25. As direct current flows from the source 20 through the resistor branch 21 and movable contact 38 of the potentiometer 31, this current at the latter point divides such that one-half flows into the bridge I0 and one-half flows into the resistor 22. The arms 30, 3|, 32 and 33 of attenuator 25 are simultaneously moved along the contacts of the resistor branches 26, 21, 28 and 29 respectively, to change arm Iluntil the precise indication of zero is attained.

If the zero vindication cannot be attained at once, the arms 30, 3|, 32 and 33 of attenuator 25 should be moved upwardly or downwardly to the extent of one contact in either direction. Now the adjustment of the movable arm 38 of potentiometer 31 will produce the zero indication exactly. Further actuation of the arms 30, 3|, 32 and 33 of attenuator 25 'in an upward direction to the extent of one contact should cause the indicationl of microamperes to occur again. This indication can be obtained precisely by adjusting the variable resistor I9a. The actuation of the arms 30, 3|, 32 and 33 of attenuator 25 in a downward directionto the extent of one contact should cause the zero indication to occur again. This means that for each contact of actuation of the arms 3U, 3|, 32 and 33 of the attenuator 25, the direct current power into the bridge I0 is varied by one predetermined amount and `th-at into theV thermistor armV I4 is varied by a lesser predetermined amount. Thus the bridge I0 is balanced for direct current at Vthe zero indication and is OIT-balance for direct current power at the indication of 100 microamperes by an amount which is equivalent to the difference between the one and lesser predetermined amounts of such power. If such balance is maintained for a relatively long period of time during which the voltage of source the bridge balance for direct current can be restored by a proper adjustment of the movable arm 38 of potentiometer 31.

In the bridge 2, I3 and I4 are provided with such values of effective resistance for ambient temperature lin the range of 0 to 120 F. and the standard direct current power supplied to the attenuator 25 as above explained, that theV one predetermined amount of direct current power applied to the bridge Ill is, say, 2 milliwatts, and the lesser predetermined amount of direct current power is, say, 1 milliwatt, applied to the thermistor arm |4. Hence, one-half of the direct currentpower applied to the bridge I!) is eiectively impressed on its thermistor arm I4. At the condition of bridge balance (zero indication on the microammeter I9) the thermistor arm I4 has, for example, an eiective resistance of ohms. Thus, as the arms 30, 3|, 32 and A33 of the attenuator 25 are actuated from contact to contact along the resistor branches '26, 21, 28 and 29 respectively, the direct currentv and voltage and thereby the direct current power into the thermistor arm I4 is varied in a plurality of steps, Equation l with a difference of 1 milliwatt from step to step, Equation 2. The lOo-microampere indication on microammeter I 9 represents the |4 in the range from lmilliwatt with its adjustable contact 38 at its minimum position to 0 milliwatt with its adjustable contact 38 at its maximum position and is calibrated for suchA range to indicate fractions of 1 milliwatt.

The resistor 22 serves'to balance the attenuator 25 soV that the load on the source 2|) and-resistor branch 21 is constant and so that the change of 'K l milliwatt of direct current power into vthe thermistor arm |4 can be eiected regardless of the position of the movable arm 38 of the poten- Both resistor branches 28 and 2 9 tiom'eter 31'.

serve to pad out the effect of the potentiometer" 20 tends to change.`

I0 of Fig. l, the bridge arms II, i

total load on the direct current source 20 remains 'constant forall positions of the movable arms 30, 3i, 32 and 33 of attenuator 25. Obviously,

the minimum amount of direct .current power is applied to the thermistor arm I4 whenthe arms 30,-3I, 32 and 33 of attenuator 25 are positioned on the lowermost contacts of theresistor branches 26, 21, 28 and 29 respectively, that is, at the highest power step Pi in Equation 1; andthe maximum amount of direct current power is applied to the thermistor arm I4 when these arms are positioned on the uppermost contacts o! the respective branches. that is, at step PNfin Equation `1. j.'

To achieve a measurement of alternating'current power the arms 30, 3 I, 32 and 33 of attenuatcr 25 are initially positioned on suchcontacts of theresistor branches 26, 21, 28 and` 29 respec- `the lowest power tivelyfas'will cause the loccurrence of the zero indication. Thenthe -plug I1 is inserted into the jack I6 to apply the source I8 of alternating current power-to the thermistor arm I4 whereupon such alternating current power further energizes the thermistor arm I4 which changes its effective resistance and thereby causes the unbalance of bridge I0. .This bridge funbalance causes the microammeter I9 to produce'an indication diierent from zero. With appropriate adjustments of the alternatingicurrentpower by the device |801, the lO-microampere indication is again produced. This means that an amount of alternating current power, equal to l milliwatt of direct current power required to eiect the same reading, is being applied to the thermistor arm I4.- This -provides .power measurement on the basis of a directy indication bylnoting either the difference between -thetwo readings on the vcalibrated attenuator 25,orthe reading on the microammeter. -For this purpose, the attenuator 25 can be calibrated in steps of 1 milliwatt as hereinafter explained.

If the amount of alternating current power supplied-to the thermistor arm I 4`is less than 1 milliwatt, the indication on the microammeter I9 will be less than 100 microamperes, but in terms of a fraction of 1 milliwatt. For example, an indication of 65 microamperes will indicate that 0.65,mlliwatt of alternating current power is being supplied to the thermistor arm I4'. If the alternating current power is more than i milliwatt but less than 2milliwatts, the indication on the microammeter I9 will be greater than 100 but less than 200 microamperes. In this connection, a reading, for example, of 165 microamperes will indicate 1.65 milliwatts of power are beingsupplied to the thermistor arm I4.

A second method of accomplishing measurementsof alternating current power involves balancing the bridge while the source I8.of alternating current power is applied to the thermistor arm I4. In this case the bridge I0 is initially balanced for direct current power to provide the zero indication as above described. Thereafter theattenuator 25 is so actuated that' its arms 3U, 3I, 32 and 33 are moved in a downward direction to the extent of one or more contacts and thereby toremove 1 or multiples of 1 milliwatt of direct Y current power from the thermistor arm I4. I'hls causes the microarnmeter I3 to indicate oli-scale, below zero. Now the source I8 `oi? alternating current power is applied to the bridge arm I4 in the manner above described, and the alternating current power adjusted until the zero indication is again restored. This means that 1 or multiples of 1 milliwatt of alternating current power is being applied to the thermistor arm I4. Thus the amount of alternating current supplied to the thermistor` arm I4 is equivalent to the amount of direct current power previously withdrawn therefrom by the attenuator 25. This is indicated by the different between the readings of the calibrated attenuator 25 at the two zero indications. Incase such alternating current power of source I8 isvless than 1 milliwatt, the microammeter I9 will provide an indication in terms of a. fraction of v1 milliwatt as mentioned above..

vA third method of accomplishing measurements of alternating current power involves inotingthe amount of direct current powervrequired to balance the bridge I0, Fig. 1, for the conditions (1) when the alternating current source I8 is disconnected from the thermistor arm I4, and (2) when the alternating current sourceI8 is applied to the thermistor arm I4. The diterence between two, such amounts of direct current power will be equal to the amount of alternating current` power applied to the thermistor arm I4.

Since the potentiometer 31 isicalibrated in the i range from 0 to 1 milliwatt as above mentioned, the attenuator 25 is also calibrated in steps of 1 milliwatt,` commencing with the minimum amount of direct current power at the highest step P1, Equation 1, and the maximum amount of direct current power at the lowest power step PN. Equation 1. To obtainthe full amount of direct current power for each of the above two conditions, the readingsi'of attenuator 25 and potentiometer 31 are added together.

In one embodiment of Fig. 1, theI circuit parameters used served to`vary the direct current power into the thermistor arm I4 in 1 milliwatt step throughithetange from '1 to 18 milliwatts, such thatthc attenuator 2E was effective over the range from? fioriti milliwatts, and the potentiometor 371 for 'tlimrange 0 to 1 milliwatt. The range of direct current power from 0 to 6 milliwatts is eiective forf'es'tablishing the resistance of the thermistor arm i4, at ohms for the balanced condition inthe ,bridge I0 whereby the zero indication is caused to.l occfr on the microammeter I9. p

What is claimed is:

An electrical circuit comprising a Wheatstone bridge having a resistance arm provided witha temperature coeflicient of resistance, a source of alternating current power connected to said arm, an indicator connected across one bridge diagonal, a source of direct current power, and an adjustable resistance network for connecting said direct current source to the .other bridge dig agonal, comprising a path connected in series".VK

withv one terminal of said direct current source and one terminal of said other bridge diagonal, said path including in sequence a first adjustable resistance branch, a movable contact, a resistor associated with said contact, and one terminator` said resistor, a second adjustable resistanceA branch connected in shunt of said direct current source, a xed resistance branch connected across a second terminal of said resistor and the opposite terminal ofsaid direct current source, and third and fourth adjustable resistance. branches. each of said last branches being inter- 9 10 posed in series with one terminal of said resistor, al1 of said adjustable resistance branches being UNITED STATES PATENTS mechanically connected together. Number Name Date 1,590,420 Chubb June 29, 1926 EDWARD W- HOUGHTON a 1,791,563 Hom Feb. 10,1931 1,039,925 Gati Oct. 1, 1912 REFERENCES CITED The following references are of record 1n the le of this patent: 

